#alert{
  box-sizing: border-box;
  position: fixed;
  left: 50%;
  top: 50%;
  width: 50%;
  border: 2px solid #000;
  background: #f1f1f1;
  transform: translate(-50%, -50%);
}
.alert-header{
  border-bottom: 1px solid #000;
  text-align: center;
  font: 20px/40px '宋体';
}